1. Field of the Invention
The present invention relates to a position detecting apparatus and method used with a projection exposure apparatus and adapted to detect position detection marks formed on a surface of a substrate such as a semiconductor wafer, and more particularly, to a position detecting apparatus suitable for using as an alignment sensor provided in an exposure apparatus for exposing a photosensitive substrate through a mask pattern (or for transferring a mask pattern onto a photosensitive substrate) in a photolithography process in manufacturing semiconductor elements, image elements (such as CCDs), liquid crystal display elements or thin film magnetic heads, for example, and adapted to detect position detection marks on the photosensitive substrate.
2. Description of the Related Art
For example, in a photolithography process for manufacturing semiconductor elements and the like (for example, a process for forming a resist image of a mask pattern on a substrate), a projection exposure apparatus (such as a stepper) for transferring a pattern formed on a reticle as a mask onto a wafer (or a glass plate and the like) on which photoresist is coated, via a projection optical system, or, another exposure apparatus (such as an exposure apparatus of proximity type) for directly transferring a reticle pattern onto a wafer has been used.
For example, since the semiconductor wafer is formed by laminating multilayer circuit patterns on the wafer in a predetermined positional relation, when two or more circuit patterns are formed on the wafer by using such an exposure apparatus, alignment between the reticle and the circuit pattern on each of shot areas of the wafer must be performed with high accuracy prior to the exposure operation. To perform such alignment, alignment marks (wafer marks) as position detection marks have already been formed on the wafer in the previous processes, and, by detecting the positions of the alignment marks by means of an alignment sensor mounted on the exposure apparatus, the accurate or correct positions of the circuit patterns in the shot areas on the wafer can be detected.
In the conventional alignment sensors, for example, it is known to use a system (referred to as "laser beam scan system" hereinafter) in which a spot-shaped or a sheet-shaped laser beam and an alignment mark are scanned relative to each other in a measurement direction to detect generated scattering light and/or diffraction light which is generated from the alignment mark, and the mark position is determined on the basis of the change in intensity of the light, or a system (referred to as "image forming position detecting system" hereinafter) in which a broadband luminous flux from a light source such as a halogen lamp is illuminated on a predetermined area including the alignment mark to obtain an image of the mark through an image forming optical system, and the position of the mark is determined on the basis of an image signal from the system. The laser beam scan system is also called a "laser-step-alignment" system LAS system and the image forming position detecting system is also called an "FIA" Field Image Alignment system.
In signal treatment methods for accurately determining or detecting the position of the alignment mark on the basis of the mark detection signal obtained based on the alignment sensor, there is a method wherein the mark detection signal curve is sliced at a predetermined intensity level and coordinates of an intersection between the signal curve and the intensity level are utilized as the mark positions, and another method wherein a relation between the mark detection signal curve and a predetermined reference signal is calculated so that a position where the relation becomes a maximum is utilized as the mark position.
Among the above-mentioned conventional alignment sensors, in the laser beam scan system, since the used detection luminous flux is a monochromatic laser beam, there may arise multi-interference between the surface of the wafer and a surface of the photoresist coated on the wafer, thereby causing a significant error in the detected mark position. To avoid this, detection luminous flux having a plurality of wavelengths (composite luminous flux obtained by combining a plurality of laser beams having different wavelengths) is used to reduce an influence of the multi-interference caused by the monochromatic feature. On the other hand, in the image forming position detecting system, since a broad-band luminous flux is used as the illumination luminous flux, no multi-interference is generated.
Recently, as semiconductor integrated circuits have become miniaturized, a process for flattening the surface of the wafer has been introduced after a film forming process and prior to the photolithography process. This flattening process provides an advantage in that an element feature can be improved by making a thickness of the film on which the circuit pattern is formed uniform and an advantage that a negative influence of an unevenness of the surface of the wafer which causes a line-width error of a transferred pattern can be reduced.
However, in systems in which the position of the alignment mark is detected on the basis of the change in the unevenness and/or the change in reflectance factor at the alignment mark portion of the wafer surface, since the degree of the unevenness of the alignment mark portion is greatly decreased by the flattening process, there arises an inconvenience that it is difficult to detect the alignment mark. Particularly, in a process regarding opaque formed films (such as metal films and semiconductor films), since the alignment mark is coated or covered by the opaque film having uniform reflectance factor, the positional detection merely depends upon the degree of the unevenness of the surface of the film, which slightly changes in accordance with the unevenness of the alignment mark, thereby causing positional detection to be more difficult it performs. That is to say, if the opaque formed film is flattened, the detection of the position of the alignment mark would become more difficult.
Since an amount of level difference of the unevenness (height difference between top and bottom levels of the unevenness) of the surface of the opaque formed film is much less than a coherent length of light defined by a wavelength and a width of the wavelength of the detection luminous flux, interference between light reflected from the top of the unevenness (level difference portion) and light reflected from the bottom of the unevenness causes a problem (although such interference does not cause any problem so long as the broad-band light is used regarding the marks having a conventional amount of the level difference). For example, if the bottom of the unevenness of the uneven mark is asymmetrical due to an inclination or the like, since interference conditions of the reflection differ from each other, at mark edges on both sides of the bottom, in between the light reflected from the top of the unevenness and the light reflected from the bottom of the unevenness, a detected signal waveform would also become asymmetric, thereby causing an error in the position detection result.